1. Field of the Invention
The invention relates to belt tensioning devices and arrangements, and in particular to a spring-actuated belt tensioner for use with an endless belt of a vehicle accessories drive system. More particularly, the invention relates to an improved, inexpensive belt tensioner construction having a unique damping mechanism, which provides a constant damping force against movement of the belt-engaging pulley in an opposite nontensioning direction by a spring-biased friction cam block arrangement.
2. Description of the Prior Art
There is the trend today in the automobile industry to operate the various vehicle accessories, such as the power steering pump, oil and air pumps, air conditioning and alternator, by a single endless belt driven by a pulley connected to the engine crankshaft. This system is referred to as a "serpentine" drive belt system. To ensure optimum operating efficiency for these various accessories, it is necessary that the drive belt be maintained at a predetermined tension to assure efficient performance of the accessories as well as satisfactory service life for the belt. Due to the relatively greater length for the single drive belt which replaces the heretofore plurality of smaller belts, there is a greater tendency for the belt to stretch which will affect the operating characteristics of the driven accessories. Therefore, it is desirable that a belt tensioning device be used for these endless belts to provide reliable service over an extended period of time and to maintain a constant amount of tension thereon regardless of the amount of belt stretch.
Numerous devices have been proposed and used to accomplish this purpose. One type of tensioner uses a bushing formed of an elastomeric material which is placed in compression by some mechanical means for continuously exerting a tensioning force on the belt. Examples of these constructions are shown in U.S. Pat. Nos. 3,975,965 and 4,144,772. These tensioner constructions, which use an elastomeric material, have the disadvantages in that the high load rate which they exert on the belt results in the rapid loss of tensioning as the belt stretches, and this load rate limits the stroke of the belt-engaged idler pulley to a shorter distance than desired. Also, sudden acceleration and deceleration and peak load frequencies at various points in the drive train primarily at low speeds can cause a belt surge and a whipping action to occur which creates a time lag before full damping is achieved.
Numerous other types of belt tensioning devices use coil springs which are either in compression or tension, for applying and maintaining the tensioning force on a belt-engaging idler pulley or chain-engaging sprocket. Some examples of these types of constructions are shown in U.S. Pat. Nos. 2,703,019, 3,413,866, 3,483,763, 3,631,734, 3,768,324, 3,812,733, 3,924,483, 3,965,768 and 4,108,013. Some coil spring-actuated devices use the biasing force of a spring in combination with hydraulic-actuated members for regulating the amount of tensioning force applied to the belt and for achieving a damping effect. Examples of these combination spring and hydraulic-actuated belt tensioners are shown in U.S. Pat. Nos. 2,051,488, 3,142,193 and 4,077,272.
It is desirable that a belt tensioner be provided with some type of damping means to prevent excessive oscillation from occurring in the spring tensioning member, and which will absorb sudden shocks to prevent a whipping action from occurring in the tensioner and drive belt. This damping action is especially critical when a coil spring is used for applying the belt tensioning force since coil springs are highly susceptible to developing natural oscillating frequencies when the counter force, which is exerted thereon by the belt, fluctuates during acceleration and deceleration of the vehicle. Such oscillating action affects the efficiency of the tensioning force applied to the belt by the coil spring and reduces belt life.
Various damping devices have been used with belt tensioners to eliminate or reduce this problem of coil spring oscillation. One type of construction uses a hydraulic fluid as the damping means, such as shown in U.S. Pat. Nos. 2,893,255, 3,964,311, 3,986,407 and British Pat. No. 336,737. U.S. Pat. No. 3,710,634 shows a belt tensioner which uses an eccentrically mounted mechanical pinion and rack arrangement which is spring biased by a leaf spring for absorbing an excessive amount of shock as opposed to providing a damping action for a spring-biased belt tensioning plunger.
Various belt tensioning devices also are provided with some type of mechanical retaining means which limits the movement of the belt tensioning member in a direction opposite to the tensioning direction, thereby assisting the tensioning member to maintain a constant tensioning force on the endless drive belt by retaining the member in its forwardmost belt tensioning position. Many of these known retaining arrangements use a spring-biased mechanical detent-ratchet retaining means. Examples of these prior constructions and arrangements are shown in U.S. Pat. Nos. 2,051,488, 2,703,019, 3,413,866, 3,631,734 and 3,812,733.
It also is highly desirable when developing a belt tensioner intended primarily for use on an automobile to devise a construction which can be produced as inexpensively as possible without sacrificing durability and efficiency since a savings of only a part of a dollar would amount to a sufficient overall savings when considering the millions of vehicles that are produced by the various vehicle manufacturers.
Many of these problems have been eliminated by the belt tensioner shown in eariler U.S. Pat. No. 4,299,584. This construction provides an inexpensive tensioner which has a damping mechanism incorporated therein which eliminates many of the above-discussed problems. This damping mechanism has a leaf spring which is mounted on either the engine mounting bracket or pulley lever and selectively engages a series of ratchet teeth formed on the opposite member. However, this leaf spring-ratchet teeth arrangement enables the lever plate to be adjusted in stepped intervals as the belt stretches. The distance between intervals is determined by the spacing between adjacent ratchet teeth. Occasionally, this may result in a less efficient operation than desired.
Therefore, the need has existed for such a belt tensioner having an improved damping arrangement which, in addition to the features discussed above, provides damping regardless of the position of the idler pulley and its mounting bracket with respect to the stationary engine mounting bracket as the belt stretches, which is not believed to be reliably achieved by any of the prior belt tensioning devices.